In Long Term Evolution (LTE), multi-antenna techniques are used to achieve improved system performance, which may include improved system capacity (e.g., more users per cell), improved coverage (e.g., larger cells), and improved service provisioning (e.g., higher per-user data rates). The availability of multiple antennas at the transmitter and/or the receiver can be utilized in different ways to achieve different objectives, such as, for example, objectives related to antenna diversity, antenna beamforming, and antenna spatial multiplexing. For example, multiple antennas at the transmitter and/or the receiver can be used to provide antenna diversity against fading on the radio channel. Multiple antennas at the transmitter and/or the receiver can be used to “shape” the overall antenna beam in a certain way, which can be referred to as antenna beamforming. For example, antenna beamforming can be used to maximize the overall antenna gain in the direction of the target receiver or to suppress specific dominant interfering signals. Multiple antennas can be used for antenna spatial multiplexing, which refers to the simultaneous availability of multiple antennas at the transmitter and receiver to be used to create multiple parallel communication “channels” over the radio interface. Antenna spatial multiplexing can provide high data rates within a limited bandwidth, which is referred to as Multiple-Input and Multiple-Output (MIMO) antenna processing.
Turning now to downlink (DL) reference signals in LTE, DL reference signals (RSs) are predefined signals occupying specific resource elements (REs) within the downlink time-frequency RE grid. LTE defines several types of DL RSs that are transmitted in different ways for different purposes. For example, a cell-specific reference signal (CRS) can be used: (1) by terminals (UEs) for channel estimation for coherent demodulation of DL physical channels; (2) by UEs to acquire Channel State Information (CSI); or (3) by UEs as the basis for measurement of cell-selection and handover. DeModulation Reference Signals (DM-RSs) are another example of a DL RS. A DM-RS can be referred to as User Equipment (UE)-specific reference signals that are intended to be used by UEs for channel estimation for coherent demodulation of DL channels. DM-RSs may be used for channel estimation by a specific UE, and then transmitted within the RBs specifically assigned for PDSCH/EPDCCH transmission to that UE. DM-RSs are associated with data signals and precoded prior to the transmission with the same precoder as data. Channel State Information Reference Signals (CSI-RSs) are another example of a DL RS. CSE-RSIs are intended to be used by UEs to acquire CSI for channel-dependent scheduling, link adaptation, and multi-antenna transmissions.
Turning now to uplink reference signals, similar to LTE DL, reference signals are also used in LTE UpLink (UL). LTE defines UL Demodulation Reference Signals (DM-RSs) and UL Sounding Reference Signals (SRSs). UL Demodulation Reference Signals (DM-RSs) are used by the base station for channel estimation for coherent demodulation of the Physical Uplink Shared CHannel (PUSCH) and the Physical Uplink Control CHannel (PUCCH). In LTE, DM-RS are only transmitted within the RBs specifically assigned for PUSCH/PUCCH transmission and span the same frequency range as the corresponding physical channel. UL Sounding Reference Signals (SRS) are used by the base station for CSI estimation for supporting uplink channel-dependent scheduling and link adaptation. An SRS may also be used for the base station to obtain CSI estimation for DL under the case of channel reciprocity.
With respect to CSI feedback in LTE, DL channel-dependent scheduling is a feature of LTE. In DL channel-dependent scheduling, the DL transmission configuration and related parameters can be selected based on the instantaneous DL channel condition, including the interference situation for example. To support DL channel-dependent scheduling, a given UE provides the CSI to the evolved Node B (eNB). The eNB uses the information for its scheduling decisions. The CSI may consist of one or more pieces of information, such as, a rank indication (RI), a precoder matrix indication (PMI), or a channel-quality indication (CQI). The RI may provide a recommendation on the transmission rank to use, or may provide a number of preferred layers that should be used for PDSCH transmission to the UE. The PMI may indicate a preferred precoder to use for PDSCH transmission. The CQI may represent the highest modulation-and-coding scheme to achieve a block-error probability of 10%, for example at most. Together, a combination of the RI, PMI, and CQI forms a CSI feedback report to the eNB. The information included in the CSI report may depend on the UE's configured reporting mode. For example, in some cases, RI and PMI do not need to be reported unless the UE is in a spatial multiplexing multi-antenna transmission mode.
A CSI report may be configured to be periodic or aperiodic by radio resource control (RRC) signaling. In some cases, CSI reporting using PUSCH is aperiodic. For example, aperiodic reporting may be triggered by downlink control information (DCI) formats, and can be used to provide more detailed reporting via PUSCH. A given UE may be semi-statically configured by a higher layer to feedback CQI, PMI, and corresponding RI, on the same PUSCH using one of various CSI reporting modes. Examples of various CSI reports modes are depicted in Table 1 below.
TABLE 1Example CQI and PMI Feedback Types for PUSCH CSI Reporting ModesPMI Feedback TypeNo PMISingle PMIMultiple PMIPUSCHWidebandMode 1-0Mode 1-1Mode 1-2CQI(wideband CQI)FeedbackUE SelectedMode 2-0Mode 2-2Type(subband CQI)Higher Layer-Mode 3-0Mode 3-1Mode 3-2configured(subband CQI)Referring to Table 1, for each of the transmission modes in Table 1, different reporting modes are defined and supported on PUSCH.
With respect to periodic CSI Reporting using PUCCH, a given UE may be semi-statically configured by higher layers to periodically feedback different CSI components (e.g., CQI, PMI, and/or RI) on the PUCCH using, for example, the reporting modes shown in Table 2.
TABLE 2Example CQI and PMI FeedbackTypes for PUCCH CSI Reporting ModesPMI Feedback TypeNo PMISingle PMIPUCCH CQIWidebandMode 1-0Mode 1-1Feedback Type(wideband CQI)UE SelectedMode 2-0Mode 2-1(subband CQI)Referring to Table 2, for each of the transmission modes in Table 2, different periodic CSI reporting modes are defined and supported on PUCCH.
With respect to three-dimensional (3D) beam systems (which can also be referred to as beamforming systems), a 3D beam system can explore both horizontal and elevation (vertical) angles. In addition, 3D beamforming can achieve a better degree of freedom as compared to traditional 2D beamforming systems that only consider horizontal angles. The 3D beamforming system may use Active Antenna System (AAS) technology to adjust antenna weights of horizontal antenna ports, and also the antenna elements in the vertical direction. A 3D beam can be characterized by a beam emission direction and a beamwidth ΔB. The beam emission direction can be described by the horizontal and elevation angles, where ψ represents the horizontal angle and θ represents the elevation angle. The beamwidth ΔB indicates how wide a 3D beam can span. In practice, a 3D beam is distinguished by its 3 dB beamwidth. Thus, to summarize, a 3D beam can be characterized by the parameters of horizontal angle, elevation angle, and beamwidth (ψ, θ, ΔB).
Referring to FIG. 1, an example 3D beam 102 is depicted. As shown, the emission direction of the beam 102 can be distinguished by the horizontal angle 104 (between the beam's projection on the x and y plane and the x-axis) and the elevation angle 106 (between the beam and z-axis).
Turning now to Full-Dimension (FD) Multiple-Input and Multiple-Output (MIMO), FD-MIMO typically includes a base station with a two-dimensional antenna array that supports multi-user joint elevation and azimuth beamforming. This may result in higher cell capacity compared to conventional systems in 3GPP release 12. In some cases, using FD-MIMO techniques, LTE systems can achieve 3-5× performance gain in cell capacity and cell edge throughput.
LTE release 10 has introduced a CSI-RS that can be used for DL channel CSI estimation for the UEs. There are up to 8 antenna ports specified in release 10 and up to 16 antenna ports specified in release 13.